A silver halide photographic material (hereinafter also referred to as "photographic material") is processed, after exposure, through the steps of development, desilvering, washing, stabilizing, etc. In these steps, a developing bath, a bleaching bath, a bleach-fix bath, a fixing bath, industrial, municipal or deionized water, and a stabilizing bath are used to process the photographic material.
Such a processing is usually conducted using a processing apparatus, e.g., an automatic processor, in which the photographic material is transferred from one to another of processing tanks respectively filled with the aforementioned processing liquids.
Of these processing steps, a step having a bleaching ability or a fixing ability necessitates a subsequent washing or stabilizing step or the like in which a fixer ingredient, the silver complex and other chemiclas generated during fixing of the silver halide emulsion layers are washed out to maintain the stability of images. Therefore, those chemicals are incorporated into processing liquids, such as wash water and a stabilizing bath, and accumulated to a high concentration. Examples of such contaminants include silver complexes from the fixing bath; iron or silver compounds from the bleach or bleach-fix bath; cerium compounds from a reducer for photomechanical proces; aluminum compounds from a hardening fixer.
If a waste water containing such contaminants, e.g., silver compounds, are discharged directly into a sewer system, it causes an environmental safety problem. Particularly, in the United States, Europe, and some areas in Southeast Asia, regulations for the discharge of silver compounds into sewer systems are getting severe in recent years.
The collection and treatment of such waste water is costly, especially in large-scale photofinishing laboratories where wash water is used in large quantities. From the standpoint of resource conservation, the recovery of metals, e.g., silver, from waste water containing these metals at especially high concentrations is important. Furthermore, in most of the countries, only authorized transporter can transport the silver-containing hazardous waste.
From these standpoints, it is strongly desired to remove and simultaneously recover silver and other metals from the waste water described above.
Conventional methods for removing silver or other metals include metal displacement, precipitation (reduction or sulfiding precipitation), adsorption onto an ion-exchange resin, electrolysis, electrodialysis, and reverse osmosis. These methods, however, have been unsatisfactory in that they have various problems such as secondary pollution, insufficient efficiency of removal, complicated procedure, high initial cost, and high running cost.
Recently, methods for recovering silver, from a processing liquid having a fixing ability or from a waste wash water have been proposed which methods use a water-soluble polymer capable of complexing with the silver, in the waste water.
JP-A-3-132656, describes an effect that a large amount of silver ions accumulated in a processing solution having a fixing ability can be efficiently and easily removed by adding the water-soluble polymer to the used fixing bath to precipitate a water-insoluble silver complex and separate the precipitate by filtration to regenerate the used fixing bath and reduce in the amount of the processing liquid to be used. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") However, this method, when applied to the treatment of a fixing bath containing a large amount of accumulated silver ions, fails in removing the silver compounds to the level below regulation limit although the method removes silver to the level that allows fixer reuse. Further, when the above method is applied to a dilute solution having a low silver content, only a small proportion of the water-solublizing group in the polymer molecule reacts with silver, resulting little silver precipitation, consequently, the removal of silver is insufficient.
In Japanese Patent Application No.5-238945, on the other hand, a method for desilvering is proposed which comprises adding a water-soluble polymer to a waste washing water to precipitate the water-soluble polymer with which silver has coordinates and then separate the precipitate with an ultrafiltration membrane to remove the silver. However, this method has had the following problems. Since an ultrafiltration membrane (UF membrane) is used for separation of fine particles of the precipitate, the UF membrane is apt to suffer clogging and should hence be replaced frequently. Thus, maintenance is troublesome. In addition, the frequent replacement of expensive ultrafiltration membranes, results in an increased running cost.